Output Frequency of CN0349

Agreed.

In an ideal world, the customer shouldn't be have to pick through every bit, there should be device drivers that handle that sort of thing.

-Mark

Well yes and no. I am writing my own drivers and external drivers would be great as a reference but they would not help me out directly. A correct datasheet on the other hand would have saved me days and weeks of work on this project. There are so many "mysteries" regarding just this frequency topic that its mind boggling to me.

From a customer/engineer point of view:

As I read through the datasheet (AD5934) for the first time, I was certain that the clock speed was at 16.776 Mhz as it was stated. There was no mention of the AD5933 and I only realized that this was nonsense after i read an answer to a different question from Snorlax . After going through my entire project and changing everything to 16 MHz (again, as stated in the datasheet) I was certain (again..) that this was the right way to do it and that it would solve my problems. There was no mention of another clock beeing connected to MCLK other than a 16 MHz one (to be fair enough, it is stated in the CN0349 datasheet, but not in a way you would remember after reading through many pages of two different datasheets) and tbh my only explanation would be, that it wouldnt sound as great if you wrote down that this "vanilla" device only manages to get up to a frequency of about 30 kHz (which i wouldnt have minded were it written down). There is NO mention (and i mean absolutely no mention) of a 1 Mhz clock changing the output code after the calculations mentioned on page 13 ff to a message greater than 24 Bit, which means you cannot "normally" (as in: described in the datasheet) write your start frequency / increment to the registers. Besides the many hours I had to put in just to fully understand what is meant by all these vague statements i had to change my code and setup mutliple times for no apparent reason.

Which is just my point. This device is great. It does what it should and it works exceedingly well. IF you know how to use it. Which requires a readable, understandable datasheet. There are many more ambiguous statements (not just regarding the frequency). It would be just great if these datasheet were written from a customer POV, so someone who bought this product could actually use it right away without finding deviations for no apparent reason.

Sorry for this rant but as you can see there was quite some steam building up. If you or any other Analog folks are willing to work these problems out, i am sure myself or even Snorlax are more than willing and able to help write these datasheet in an understandable way to improve the customer experience.

Anyways, thanks for all your help again. It seems to work for now and if I stumble upon any more problems, I will contanct you guys again.

Have a great day,

Muzot.

MarkThoren said:

there should be device drivers that handle that sort of thing.

The overwhelming majority of users corresponding in this forum on this subject are building their systems with standalone microcontrollers, Arduinos, etc. directly communicating with the chip. Very few are using the evaluation board - there seem to be no use cases for these chips that would involve PC. Some look for ways of hooking up Arduino I2C lines directly to chip bypassing the board controller of the eval board and, perhaps, sidestepping driver issues.

Muzot said:

Sorry for this rant

No need to be sorry - it is important to provide honest feedback. Many forum participants seem to give up on their projects  based on these chips after similar experience. It is extremely frustrating typing in the higher frequency only to see the chip generating a lower one because the the resulting codes were longer than 24 bit and got truncated at the top without a warning and this behavior not mentioned in the documentation. Imagine how many more outside this forum tried and gave up - not everyone has your stamina to wrestle with this for weeks and write a page-long assessment. 

Best of luck with the rest of your project, hopefully it is going to be smoother sailing from here. Please keep in touch in case some underwater rocks come up.

Well yeah exactly. I am going through posts on this site for quite some time now and i cannot understand how the datasheet was never updated altough so many people stumble upon the same problems...

Anyways, one question occured after changing my setup/code. For some reason the real/imaginary data i read out after reaching the last increment point (Frequency sweep complete) is always wrong and thats the case for every voltage. Doesnt matter which frequency is the last measurement point. I can easily work around that by programming one more increment number into the register so it never reaches the "real" last frequency point. Would still be interesting to me why this problem occures.

Edit:

Thats probably not the real problem. It seems like the real/imaginary data i can read out after increasing the frequency thorugh the sweep options changes in a static way. What I mean by that is that no matter where i start, I get the same results. Always starts with the desired cell constant of my conductivity probe and then the cell constant slowly decreases (i wrote an algorith which goes through all frequencies and voltages to find the optimal setup). Initally i thought this is an effect of the change in frequency which obiously changes the measured resistance in sulution but the fact that it doesnt matter at which frequency i start/stop is just strange.. Doesnt even matter what the amount of frequency increment is.

As for my routine:

1. Calibrate the GF/NOS etc for both realms and all voltages

2. Send frequency data

            measure temperature

3. Initialize (standby, init, start)

4. read out data (check res register, get res)

5. increase frequency

6. repeat frequency for all voltages

7. start at 4 again

Is there something fundamentally wrong with this routine?

I checked if the output frequencies/voltages are right multiple times and they are. Can't quite figure this one out.

There seems nothing wrong with the routine. Conductivity measurements can be tricky as the measurement cells tend to drift, especially with aqueous solutions. Hard to give any useful recommendation not knowing enough about your particular setup. As you apply different excitation voltages across your cell, depending on the nature of your electrodes at higher voltages you might trigger some electrochemical reactions that are likely to affect your results.
If you have access to an oscilloscope - as your circuit goes through the measurement routine - it is always a good idea to check the voltage waveform at the AD5934 RFB pin and at the output of U2B OPAMP - those should nearly identical (ignoring 180º phase) well-formed sinusoids without any distortion or clipping at all frequencies and voltages. 
Generally speaking, it is useful to keep some linear "equivalent load" with the impedance similar to the expected impedance of your cell, usually a resistor or a sequential RC. If the results from the cell look strange, it is always useful to connect this "load" and see if the circuit performs as expected with the load, then the issues are with the cell and not the electronics.
Feel free to post your data through Insert- > Image/video/file as a text or csv file if you think it would be useful for somebody to look at it. Re and Im data as it comes from the chip, prior other calculations, would be particularly useful.

Thank you for your reply! As this is a work project I cannot disclose much more than I already have.

What exactly do you mean by equivalent load? Would a precision resistor be enough? I have not yet tried that for some reason, but I will do so today or tomorrow and tell you how it went.

I will also check the outputs of the RFB and OPAMP output Pin. If these waveforms do not match or do not look good, what exactly would be my conclusion? Something wrong with the chip itself?

As for the voltage part and electrochemical reactions, it is not entirely unprobable but I would highly doubt effects like these taking place in standard KCl solutions. Nethertheless, I will start with a measurement with the low voltages (0.2 and 0.4 V) to test this.

Thanks again.

Ok so i checked if everything went fine with a resistor connected to the pins for the cell and i had no problems measuring the right value across the entire voltage/frequency/realm range. After doing that i connected my cell again and lo and behold, it worked fine. Probably a faulty cable of something like that.

So the problem is fixed for now / never existed.

Muzot said:

What exactly do you mean by equivalent load?

Yes, 1% or better yet 0.1% resistor would be a good "load" to make sure that the electronics is functioning as expected when encounter strange behaviour from the real-life measurements cell. Depending on the construction of your cell you might want to use a sequential RC circuit that would have complex-valued, frequency-dependent impedance (the resistor is nearly 100% real-valued and frequency-independent impedance) that could reflect some of the frequency-dependent behaviour of a real measurement cell.

Muzot said:

If these waveforms do not match or do not look good, what exactly would be my conclusion?

If you see sine wave clipped at the top and/or bottom - this means that the system has too much gain due to too low impedance of your measurement cell - reducing the value of Rfb resistors to get the sive wave back in linear range would be required. If you see that the sine wave has a distorted shape, for example, starting to resemble triangular or saw-tooth waveform - the response form your measurement cell is non-linear while the impedance is supposed to be linear, so the measurement results are not quite valid (typically these would not be visible on the oscilloscope, one would need to measure higher harmonics content in this waveform with a spectrum analyzer). If the waveform is "hairy" with lots of high-frequency noise and spikes - the measurement setup is picking up too much environmental noise and interference, so better shielding is required.

Muzot said:

effects like these taking place in standard KCl solutions.

If you have electrodes with metal surfaces exposed to the solution it is nearly certain that you trigger some electrochemical reactions if your excitation voltage as above several hundred millivolts peak-to-peak.